How Do You Calculate Dew Point and Relative Humidity

There are two main approaches to calculate dew point and relative humidity:

1. Using a psychrometer or hygrometer:

These instruments directly measure humidity and some models can also provide dew point readings.

2. Using mathematical formulas:

There are a couple of formulas you can use to calculate dew point and relative humidity if you know the air temperature and either the relative humidity or the dew point.

• Calculating dew point from temperature and relative humidity:

A relatively simple and accurate formula for dew point (Td) in degrees Celsius is:

Td = T – ((100 – RH)/5)

where:

• T is the air temperature in degrees Celsius
• RH is the relative humidity in percent

This formula is most accurate for higher relative humidity values (above 50%).

• Calculating relative humidity from temperature and dew point:

The equations to calculate relative humidity (RH) are more complex and involve exponential terms. Here’s one option:

RH = 100 * (EXP((17.625 * Td) / (243.04 + Td)) / EXP((17.625 * T) / (243.04 + T)))

where:

• Td is the dew point temperature in degrees Celsius
• T is the air temperature in degrees Celsius
• EXP is the mathematical exponent function (available on most calculators)